Method and apparatus for code assignment in a spread spectrum wireless communication system

ABSTRACT

Method and apparatus for code assignments in a spread-spectrum wireless communication system incorporating a Large Area Synchronized-Code Division Multiple Access (LAS-CDMA) protocol. The method determines a set of LS codes based on an interference free window size. An arborescence structure provides the correspondence between the interference free window size and the set of LS codes. Subsets of the LS codes are formed having null cross-correlation within the interference free window. The subsets are assigned to neighboring cells in the system to reduce interference between neighbors. In one embodiment, a controller determines the subsets and makes the assignments to cells within the system.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

The present application for patent is a Continuation and claims priorityto patent application Ser. No. 09/737,893 entitled “METHOD AND APPARATUSFOR code assignment in a spread spectrum WIRELESS communication system,”filed Dec. 15, 2000, now U.S. Pat. No. 6,714,526 now allowed, andassigned to the assignee hereof and hereby expressly incorporated byreference herein.

BACKGROUND

1. Field

The present invention relates to wireless data communication. Moreparticularly, the present invention relates to a novel and improvedmethod and apparatus for code assignment in a spread-spectrum wirelesscommunication system.

2. Background

In a spread spectrum wireless communication system, a base stationcommunicates with multiple mobile users. In one system, a Code DivisionMultiple Access (CDMA) system, such as specified in the “TIA/EIA/IS-95Mobile Station-Base Station Compatibility Standard for Dual-ModeWideband Spread Spectrum Cellular System,” hereinafter referred to as“the IS-95 standard,” or the “TIA/EIA/IS-2000 Standards for cdma2000Spread Spectrum Systems,” hereinafter referred to as “the cdma2000standard,” codes are applied to the data and control information. Thecodes identify the target recipient as well as the sender. Operation ofa CDMA system is described in U.S. Pat. No. 4,901,307, entitled “SPREADSPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE ORTERRESTRIAL REPEATERS,” and also in U.S. Pat. No. 5,103,459, entitled“SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONESYSTEM,” both assigned to the assignee of the present application forpatent and hereby expressly incorporated by reference. The forward linkfrom base station to mobile users assigns a unique Walsh code to eachmobile with which it transmits, wherein the Walsh code identifies themobile. Similarly, the reverse link from the mobile user to the basestation uses a Pseudorandom Noise (PN) code for channelization.

Alternate spread spectrum systems incorporate a variety of codes for thetwo-way identification. A problem exists as codes may be reused inneighboring cells and/or sectors, creating interference for adjacentneighbors. Therefore, a method is needed to provide the two-wayidentification of wireless communications while minimizing and/orreducing the interference experienced by neighboring cells and/orsectors. Similarly, there is a need for a code assignment method thatachieves that reduces neighbor interference.

SUMMARY

The disclosed embodiments provide a novel and improved method for codeassignment in a spread spectrum wireless communication system. In oneaspect, in a wireless communication system adapted for Large AreaSynchronized-Code Division Multiple Access (LAS-CDMA) transmissionshaving a plurality of LS codes, a method includes determining a size ofan Interference Free Window (IFW), calculating a plurality of subsetsfrom the plurality of LS codes, each subset comprising LS codes as afunction of the IFW, assigning a first of the plurality of subsets to afirst portion of the system, and assigning a second of the plurality ofsubsets to a second portion of the system.

In another aspect, a wireless communication system incorporating anLAS-CDMA protocol includes a first cell, the first cell being assigned afirst subset of LS codes; and a second cell, the second cell beingassigned a second subset of LS codes, wherein the first and secondsubsets have a null cross-correlation within a predeterminedinterference free window. Note that the LS codes within the first andsecond subsets also have a null cross-correlation with a predeterminedIFW that is typically wider than that between cells.

In another aspect, in a Large Area Synchronized-Code Division MultipleAccess wireless communication system, a method includes transmitting afirst communication within a first cell, the first communicationidentifying at least one mobile station within the first cell by a firstLS code within a first subset of LS codes; and transmitting a secondcommunication within a second cell, the second communication identifyingat least one mobile station within the second cell by a second LS codewithin a second subset of LS codes, wherein a cross-correlation of thefirst and second subsets is null within an interference free window.

In another aspect, a controller in a Large Area Synchronized-CodeDivision Multiple Access wireless communication system includes a firstset of instructions for determining an interference free window size forcommunications between neighboring cells; and a second set ofinstructions for determining at least two sets of LS codes based on theinterference free window size, a first set for identifying mobilestations in a first cell and a second set for identifying mobilestations in a neighboring cell, wherein the two sets of LS codes have anull cross-correlation within an interference free window. This is alsotrue of the LS codes within each of the subsets with respect to eachother.

According to still another aspect, in a Large Area Synchronized-CodeDivision Multiple Access wireless communication system, the systemhaving a first cell and a first cell neighborhood, a computer datasignal is embodied on a carrier wave, the signal including a firstportion comprising information for transmission to a first mobile userin a first cell; and a second portion comprising a first LS codecorresponding to the first mobile user, wherein the first LS code ispart of a first set of LS codes exclusively assigned to the first cellwithin the first cell neighborhood.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the presently disclosed methodand apparatus will become more apparent from the detailed descriptionset forth below when taken in conjunction with the drawings in whichlike reference characters identify correspondingly throughout andwherein:

FIG. 1 illustrates in block diagram form a wireless communication systemaccording to one embodiment;

FIG. 2 illustrates in block diagram form a protocol for transmittingdata in a wireless system according to one embodiment;

FIG. 3 illustrates in tabular form and timing diagram form pulse codeposition assignments for LA codes in a Large Area Synchronized (LAS)code system according to one embodiment;

FIG. 4 illustrates various Interference Free Windows (IFWs) in a LAScode system according to one embodiment;

FIG. 5 illustrates a permutation table of LA codes in a LAS code systemaccording to one embodiment;

FIG. 6 illustrates an arborescence structure for determining acalculation of LS code length as a function of Interference Free Window(IFW) size;

FIG. 7 illustrates transmission protocols for mobile units in a LAS codesystem according to one embodiment; and

FIGS. 8A and 8B illustrate in block diagram form various code assignmentschedules for a LAS code system according to alternate embodiments.

DETAILED DESCRIPTION

In many spread-spectrum communication systems codes are applied totransmission signals for channelization. Often a first type of code isapplied to the data signals to identify the designated mobile user, anda second type of code is applied that is specific to the base station. ACDMA system, for example, has a large number of codes available forspreading the data and control information transmitted in a wirelesssystem, allowing one base station to communicate with multiple mobileusers. The CDMA codes include a Walsh code assigned to each mobile userand a Pseudorandom Noise (PN) code specific to the base station. Bothcodes are applied to data signals transmitted by the base station. A setof Walsh codes is a set of orthogonal binary sequences, wherein thecross-correlation over time is zero. The Walsh codes are generated usinga Hadamard matrix, wherein recursion allows expansion of a base code, orseed, into lengthier codes, thus increasing the size of the Walsh codeset.

However, in contrast to Walsh codes, PN codes do not requiresynchronization such as used to implement Walsh codes. Rather, PN codesmay be generated by linear feedback shift registers, wherein binary bitsare shifted through the different stages of the register. Output bitsfrom the last stage form the PN codes. PN codes have an auto-correlationcharacteristic that allows codes to align at the receiver. Thecombination of Walsh and PN codes allows identification of the basestation and mobile station for wireless transmissions. Other systems mayincorporate other codes and/or combinations of Walsh, PN, and othercodes to identify the mobile user and the base station.

Large Area Synchronized-Code Division Multiple Access (LAS-CDMA) Systems

In another type of spread spectrum wireless communication system,referred to as a Large Area Synchronized-Code Division Multiple Access(LAS-CDMA) system, specially designed codes, referred to as “LS” and“LA” codes, are used to spread the signals for transmission. LAS-CDMA isa technique for spread-spectrum and time-division multiple access thatcreates channels through the use of time division and orthogonal codes.One LAS-CDMA system is described in the “Physical Layer Specificationfor LAS2000,” a candidate proposal to the China WirelessTelecommunication Standard (CWTS).

In a LAS-CDMA system, communications are transmitted in frames, whereina fixed number of sub-frames are included in each frame. Each sub-frameis then segmented into a predetermined number of time slots. The LS andLA codes are used for channelization, as in other CDMA systems, whereinthe LS and LA codes are designed to have a small or zerocross-correlation over time.

A first type of code, the LS code, is applied to the data or symbols ineach time slot. The LS code identifies the mobile user that is thetarget of the transmission. Within a given cell and/or sector, differentLS codes are applied to each mobile user. In preparing eachtransmission, the appropriate LS code is modulated by a symbol, asdescribed hereinbelow, to form a symbol modulated LS code.

A second type of code, the LA code, identifies the cell and/or sector,i.e., the base station operating within the area. Unlike the LS codethat is applied to each time slot, the LA code is applied to the entiresub-frame, and defines the time allotted to each segment of thesub-frame. Application of a given LA code to the entire sub-frameresults in various sized time gaps between modulated symbols. Thecombination and order of the gap sizes serves as an identification ofthe cell and/or sector.

An exemplary embodiment of a LAS-CDMA system is illustrated in FIG. 1.The system 10 is a LAS-CDMA system including a plurality of cells 20,30, 40, 50, 60, 70, each having a base station 22, 32, 42, 52, 62, 72,respectively, for communication with mobile stations 24, 34, 44, 54, 64,74, within the system 10. A forward channel is used for transmission ofdata from base stations 22, 32, 42, 52, 62, 72, to mobile stations 24,34, 44, 54, 64, 74, within system 10. Each mobile station 24, 34, 44,54, 64, 74, uses a reverse channel for transmission of data to at leastone of base stations 22, 32, 42, 52, 62, 72. In other LAS-CDMA systems,the cells may be as configured in FIG. 1, or may be positioned in analternate manner, wherein each base station is associated with at leastone cell and/or sector.

As illustrated in FIG. 1, a first portion of the system 10 is referredto as cell 20. Other portions of the system 10 that are located in closeproximity to cell 20 make up a neighborhood of cell 20. In system 10,the illustrated portion of the neighborhood of cell 20 includes cells30, 50, and 60. Other portions within the neighborhood of cell 20 arenot shown, but may be located anywhere in close proximity to cell 20. Inone embodiment, a cell neighborhood includes those cells that border thecell geographically. In alternate embodiments, the neighboring criteriathat determines which portions are included in a neighborhood may bemade according to the interference experienced between cells, or someother criteria that relates to the interaction of wireless transmissionswithin cells.

Within system 10, signals are prepared for transmission by formattingthe signals into frames of a predetermined protocol. FIG. 2 illustratesone embodiment of a forward channel protocol 100 used in system 10 fortransmitting data and control information in 20 ms frames, wherein eachframe 102 includes a header field 104 and multiple sub-frames 106. Theheader field 104 designates pilot bursts and control information for thesync word and the sync sub-channel. The header field 104 is followed bya sequence of nine sub-frames 106, numbered 0, 1, 2, . . . 8. Thesub-frames 106 are evenly spaced within the frame 102. Note that theheader field 104 is made up of a first number of chips and each of thesub-frames 106 is made up of a second number of chips, wherein thesecond number is not necessarily equal to the first number. As usedherein, a chip is defined as the sample in time.

As illustrated in FIG. 2, each sub-frame 106 includes a predeterminednumber of time slots 108, wherein each time slot 108 corresponds to anLS code modulated by a symbol. In the present embodiment, each sub-frame106 includes 17 time slots, labeled 0, 1, 2, . . . 16. Each time slot108 includes a modulated LS code and a subsequent gap 110 of variablesize defined by the LA code. Note that the LS code is assigned to themobile user, while the LA code is assigned to the base station. Themodulated LS code is composed of a pair of complementary components,identified as “C” and “S” components as discussed in detail hereinbelow.Each C component is 64 chips and is preceded by a 4 chip gap. Each Scomponent is 64 chips and is preceded by a 4 chip gap. Therefore, eachmodulated LS code consumes 136 chips: (4+64+4+64). The minimum size ofany time slot 108 is, therefore, 136 chips.

In the present embodiment, the time slots 108 are not assigned a commonchip length, but rather each time slot 108 within a sub-frame 106 isassigned a unique chip length. The chip length of each time slot 108 isdetermined by the gap 110 appended to the modulated LS code, wherein thesizes of the gaps 110 are not uniform over the entire sub-frame 106. Thesizes of gaps 110 are determined by the assigned LA code. The LA codedefines the size of each gap 110 within the time slot 108. The LA codeis effectively a pattern that is applied to the entire sub-frame 106. LAcodes are described in detail hereinbelow.

In one embodiment, the frame 102 is composed of 23,031 chips. The headerfield 104 is assigned 1545 chips, and each sub-frame 106 is assigned2559 chips. As each sub-frame 106 contains 17 time slots 108, and eachtime slot 108 includes an LS code of 136 bits and a variable size gap110, each time slot 108 is, therefore, at least 136 chips long,sufficient for the LS code. Note that if a time slot 108 is equal to 136chips, there is no gap 110.

LA Codes

The gaps 110 are determined by the LA code of the cell and/or sector.Specifically, a LA code is a code used to determine the boundaries oftime slots within a sub-frame and to identify a cell and/or sector. LAcodes may be designated by the following parameters, (N, K0, K):

-   -   (i) “N” is the number of pulses;    -   (ii) “K0” is the minimum pulse interval; and    -   (iii) “K” is the total LA code length in chips.

A pulse is a function with unit energy and infinitesimal duration. Eachpulse identifies a time boundary for a time slot 108. The minimum pulseinterval is determined by the size of the LS code. In the presentembodiment, the LS code consumes 136 chips, and therefore the minimumpulse interval is 136 chips. Alternate embodiments may implement LScodes with different lengths and hence different minimum pulseintervals.

As the LA code defines the timing boundaries over the entire sub-frame106, the total LA code length refers to the length of the sub-frame 106,including all time slots 108. In the present embodiment, the total LAcode length is 2559 chips. Note that the LA code is a parsing schemethat covers the entire sub-frame 106; and therefore, the total LA codelength is defined as the length of the sub-frame 106, and not the lengthof the LA code that lists the time intervals assigned to each time slot108 within the sub-frame 106. With respect to FIG. 2, while one LS codeis applied to each time slot 108, one LA code is applied to the entiresub-frame 106.

As illustrated in FIG. 2, each sub-frame 106 is 2559 chips, wherein achip is defined as the sample after spreading. The total LA code length,K, is given as the length of one sub-frame 106. Each sub-frame 106 issegmented into a predetermined number of segments, each segmentallocated sufficient chips to accommodate one LS code. The number ofpulses, N, corresponds to the number of segments after segmentation. Forexample, as illustrated in FIG. 2, each sub-frame 106 is segmented into17 segments, i.e., N=17. The minimum pulse interval, K0, is measured inchips and is constrained to a length of at least equal to one LS code,i.e., 136 chips. While K0 defines the minimum segment length of a timeslot 108, not all time slots 108 are equal to the minimum segmentlength, and perhaps no time slot 108 is equal to the minimum segmentlength. Each time slot 108 is assigned a segment length. When thesegment length of the time slot 108 is greater than 136 chips, thedifference is made up by gap 110 subsequent to the S component of themodulated LS code.

In the present embodiment, LA codes are given having parameters(17,136,2559), i.e., N=17; K₀=136; and K=2559. An exemplary LA code isillustrated in FIG. 3. Seventeen pulse intervals are assigned indexvalues from 0 through 16, each corresponding to one of the 17 time slots108 of FIG. 2. Each pulse interval has an associated chip length andpulse position in time with respect to the other pulses. The pulsepositions in time for each index value are illustrated below the LAcode. The corresponding pulse positions for the LA code illustrate thechip beginning of each time slot 108. Within the LA code, from the left,the first pulse interval corresponds to a first time slot 108 of FIG. 2(labeled “0”). The first pulse interval is equal to 136 chips, or theminimum pulse interval. Therefore, the first time slot 108 has a gap 110equal to zero. Continuing with the LA code, the next pulse interval isassociated with the second time slot 108, labeled “1.” This interval is138 chips, and therefore the corresponding pulse occurs at(136+138)=274. The second time slot 108 has a gap 110 equal to twochips. Successive pulse intervals within the LA code define the timingboundaries of the time slots 108 within sub-frame 106, wherein the finalpulse occurs at the time corresponding to the 2559th chip. Within the LAcode of the illustrated embodiment, successive pulse intervals are twochips greater than previous pulse intervals, until the end of the LAcode. Alternate embodiments may implement an alternate assignment ofpulse intervals. The combination and order of pulse intervals creates apattern. The pattern is then permuted to create multiple patterns,providing distinct LA codes applicable to multiple cells and/or sectors.

FIG. 4 illustrates an exemplary permutation table for 16 permutations ofthe primary LA codes of FIG. 3. Each permutation defines a combinationof interval index assignments of the primary code, wherein eachpermutation alters the arrangement and/or order of intervals. The LAcode permutations identify cell and/or sector of the base station. LAcodes are used in LAS-CDMA to create separate multiple accesstransmission channels on a same RF carrier. The channels may be used bydifferent cells and/or sectors.

LS Codes

While LA codes are used to identify cell and/or sector, LS codes areused in LAS-CDMA to spread the transmitted signal and create multiplecode divided transmission channels. As illustrated in FIG. 2, an LS codeis a complementary orthogonal code pair used to spread symbols. Eachoccurrence of an LS code consists of a pair of codes of equal length: aC component and a S component. The C and S components are separated bygaps. For each LS code, constituent C and S components are designed suchthat the cross-correlation function of the LS codes is zero within acertain duration.

In one embodiment, the C and S components are generated from initialseeds. Example seed pairs may be given as follows:

-   -   (C1; S1)=(++; +−)    -   (C2; S2)=(−+; −−),        wherein a “−” indicates a low logic level bit and a “+”        indicates a high logic level bit. In one embodiment, a seed set        contains two such seed pairs. The following rule is used to        generate double length codes from a seed pair:

(C1   C2; S1   S2) (C1 −C2; S1 −S2) (C2   C1; S2   S1) (C2 −C1; S2 −S1),wherein a negative indicates the binary complement of the original. Therule may be used to generate continuously longer codes.

The present embodiment generates subsets of LS codes within each set ofLS codes having a given LS code length. The subsets are designed suchthat the cross-correlation function between any two LS codes in thesubset is zero. The range of offsets within which the zero valuedcross-correlation is maintained is referred to as the Interference FreeWindow (IFW). The range or interval of the IFW is represented as [−d,+d], wherein “d” represents the offset distance from the origin, andwherein the origin is the position where the two LS codes are exactlyaligned. The range is described over the offset origin, wherein theoffset origin refers to no offset between the codes. Thecross-correlation properties of the LS codes allow for Multiple AccessInterference (MAI) rejection and/or reduction.

Another characteristic of the LS codes is that the auto-correlation isnull within an interval centered around the offset origin, but not atthe origin. The auto-correlation functions are generally defined for asequence x, as:

${{R_{x}(i)} = {\sum\limits_{j = 0}^{J - 1}\;{x_{j}x_{j - i}}}},$wherein J is the number of elements in each occurrence of the sequenceor code, x is a code element, and j is the code element index. For eachsuccessive shift i, the auto-correlation function calculates thesummation of the product of x_(j) and its shifted version x_(j-i). Theauto-correlation property of the LS codes allows rejection and/orreduction of the multipath-induced Inter-Symbol Interference (ISI).

The number of LS codes of a given LS code length that satisfy thecross-correlation and auto-correlation properties corresponds to a givenIFW size. In other words, the number of LS codes per subset may bedetermined by the IFW size and the LS code length. For a fixed LS codelength, the width of the IFW increases, the number of LS codessatisfying these properties decreases.

FIG. 5 illustrates several IFWs with respect to offset intervals. TheIFW defines the offset between the two codes, wherein thecross-correlation between the two codes is null. The distance from theorigin corresponds to the shift from the origin, wherein the codes areshifted in time with respect to each other. As illustrated, forIFW=[0,0], the window covers the origin. As the window increases, thenumber of LS codes available decreases.

In one embodiment, LS codes are defined according to an arborescentstructure, such as illustrated in FIG. 6. The structure begins with acode length of two bits, wherein each of the C and S components are onebit. Progressing down the structure, four LS codes are provided with twobit components, followed by eight LS codes of four bit components, andfinally through to 128 LS codes of 64 bit components. For a 128 bit LScode, the desired IFW determines the number of codes available to form aLS code subsets. For example, an IFW=[−7,+7] the subset includes 16codes. As the size of the window decreases, the number of codesavailable for the subset increases. For IFW=[−3,+3] the subset includes32 codes; for IFW=[−1,+1] the subset includes 64 codes; and forIFW=[0,0] the subset includes all 128 codes. Note that IFW=[0,0]indicates that there is effectively no IFW.

The following table provides an example for LS code length of 128 bits.

TABLE 1 IFW versus LS Code Subset Size Number of IFW LS Codes [0, 0] 128[−1, +1] 64 [−3, +3] 32 [−4, +4] 16Signal Transmission

FIG. 7 illustrates transmissions within each of two cells within system10 of FIG. 1. The transmission signals include symbols 108, asillustrated in FIG. 2, separated by gaps in time of varying duration. Inthe example illustrated, signals are transmitted to mobile user 24within a first cell that is assigned a first LA code. The first LA codedefines a time boundary sequence. The first LA code defines the chipsizes of gap 1, gap 2, etc. For mobile user 34 within a second cell ofthe first cell's neighborhood, signals are transmitted having a secondLA code defining chip sizes of gap 3, gap 4, etc. The LA sequence foreach mobile user 24, 34 continues with other gap durations betweenmodulated LS codes. The particular LA sequence identifies the basestation from which the transmissions are sent. The gap between symbolsis a time duration during which no information is sent. The sequence ofgaps, or the LA sequence, is effectively a code assigned to a basestation or other transmitting source. The total frame length for each ofmobile user 24 and mobile user 34 is consistent; only the time durationand order of the gaps between modulated LS codes is different amongusers. Each frame complies with a predetermined format and/or protocolthat defines the time allowed for an entire frame. The LA codeeffectively sets the symbol boundaries within a frame. Note that fortransmissions from a given base station, the sub-frame pattern, i.e., LAcode, repeats in successive frames. Alternate embodiments may alter theframe length per user. Note that within a cell and/or sector, a basestation uses a single LA sequence. Neighboring cells will be assigneddifferent LA sequences.

Within a cell and/or sector each mobile user is assigned a unique LScode. However, neighboring cells/sectors may reuse the LS codes, andmost likely will have at least one LS code in common. The common use andreuse of LS codes by neighboring cells and/or sectors createsinterference that may impede correct reception of transmitted signals byone or multiple mobile users in neighboring cells/sectors. The exemplaryembodiment utilizes the IFW characteristics of a LAS-CDMA system byforming subsets of codes based on a desired IFW.

Assignment of LS Codes

FIG. 8A illustrates assignment of LS code subsets to neighboring cellswithin a system 700 having cells 702, 704, 706, 708, 710, 712. Asillustrated, a unique LS code subset is assigned to each neighboringcell. For example, a subset A is assigned to cell 702. The neighbors tocell 702 are each assigned a different and unique subset of LS codes,wherein subset B is assigned to cell 704, subset C is assigned to cell706, subset D is assigned to cell 708, subset E is assigned to cell 710,and subset F is assigned to cell 712. In this way, no mobile user incell 702 will be assigned a code used in a neighboring cell. There is noreuse within the neighboring cells of cell 702.

Note that not all neighbor cells of cell 702 may be assigned differentsubsets. One assignment schedule is illustrated in FIG. 8B, wherein notwo neighbor cells have a common LS code set assignment. The illustratedschedule assigns three LS code subsets to the cells within system 700.The subset A is assigned to cell 702. The subset B is assigned to cells704 and 708, as these two cells are not neighbors. Subset C is assignedto cells 706 and 710. Note that subset A is then assigned to cell 712,as this cell is not a neighbor of cell 702. As in the scheduleillustrated in FIG. 8A, no neighboring cells are assigned a common codeset.

In accordance with one embodiment, a process of assigning LS codesubsets determines the size of the IFW. This is the acceptable level ofinterference allowed in the system. For example, an IFW=[0,0] indicatesthat no offsets maintain the desired cross-correlation property. Inother words, there is no window of offsets that is interference free.Similarly, an IFW=[−1,+1] indicates that a group of LS codes can befound that maintains these properties for offsets of +1 to (−1). Thesize of the LS subset is calculated as a function of the IFW selected.Specific code subsets are formed, wherein the number of codes availablefor a subset is determined from the selected IFW and the length of theLS codes. The LS codes may be assigned to subsets sequentially or insome other fashion, wherein in one embodiment, at least three subsetsare formed providing complete isolation of cells with a honeycomblayout, as illustrated in FIGS. 8A and 8B. Alternate embodiments mayincorporate alternate numbers of subsets and allow different isolation.Still other embodiments may incorporate additional subsets to provideadditional isolation of cells. Each system may be implemented toaccommodate the vagaries of the transmission environment. For example,one system may require less isolation due to a sparse concentration ofexpected mobile users or due to a specific type of terrain, while othersystems may require additional isolation beyond neighboring cells due toa dense concentration of users. As an example, consider the caseillustrated in FIG. 6, for a total of 128 LS codes, wherein a desiredIFW=[−1,+1] corresponds to 64 LS codes per subset. The available codesmay then be assigned to subsets.

In one embodiment the system 10 selects the IFW based on the multi-pathpropagation delay profile of a mobile user. Note that the MAI and ISIwithin the IFW may be reduced. It is more difficult to remove or rejectthe MAI and ISI outside of the window. As the size of the IFW increases,the number of LS codes that maintain the IFW decreases. However, thenumber of LS code subsets increases. For example, as discussedhereinabove, with an LS code length of 128 bits, an IFW=[−1,+1] resultsin subset size of 64 LS codes. As there are a total of 128 codes, thisallows two subsets of 64 LS codes. For an IFW=[−3,+3], the resultant LScode subset size is 32, and there are, therefore, four subsets that maybe formed from the 128 LS codes.

A LS code subset is then assigned to one cell. A check is made to see ifthe same code subset is used in a neighboring cell. This is to ensurethat neighboring cells are using distinct subsets. If no neighboringcell is using the same subset, processing continues to determine if allcells have been assigned. If a neighboring cell uses the same LS codesubset, processing continues to select a different LS code subset. Thedifferent LS code subset may be the next sequential subset of codes ormay be a disjoint set of codes. The processing continues until all cellsin the system have been assigned code subsets.

The process will typically be performed by a base station controller(not shown) that provides control for multiple base stations within awireless telecommunication system. The processing may alternately beperformed by each base station, wherein each base station within asystem, such as stations 22, 32, 42, 52, 62, 72, communicates with eachother base station in the selection of LS code subsets. In such anembodiment, once a base station has selected a LS code subset, thisselection is communicated to other base stations within the system.Negotiation between base stations may be used to allow each base stationto isolate communications within the corresponding cell from neighboringcells, i.e., the neighborhood, while allowing each neighbor to performthe same with respect its neighborhood. The assignment of LS codesubsets is a dynamic process and may be modified or changed due tochanging circumstances within a given system. When for example, thedesired IFW changes, it may be necessary to change the LS codeassignments. In this case, fewer codes may be available to form subsets.In another instance, the concentration of mobile users may change,requiring a smaller IFW and making more codes available.

According to one embodiment, a wireless LAS-CDMA communication systemimplements multiple LS code subsets, wherein each LS code subset isassigned to a cell and/or sector within the system and different subsetsare assigned to neighboring cells. The subsets are formed to achieve adesired IFW, wherein the IFW determines the number of codes in eachsubset. Application of such subsets to neighboring cells reduces theinterference between neighboring cells. In one embodiment, the size ofthe subsets is determined by the length of the codes and the IFWaccording to an arborescence structure. In this way, the interferenceexperienced by neighboring cells in a wireless communication system isreduced.

Thus, a novel and improved method and apparatus identifying mobile usersin a wireless transmission in a wireless communication system has beendescribed. Those of skill in the art would understand that the data,instructions, commands, information, signals, bits, symbols, and chipsthat may be referenced throughout the above description areadvantageously represented by voltages, currents, electromagnetic waves,magnetic fields or particles, optical fields or particles, or anycombination thereof.

Those of skill would further appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the embodiments disclosed herein may be implemented aselectronic hardware, computer software, or combinations of both. Thevarious illustrative components, blocks, modules, circuits, and stepshave been described generally in terms of their functionality. Whetherthe functionality is implemented as hardware or software depends uponthe particular application and design constraints imposed on the overallsystem. Skilled artisans recognize the interchangeability of hardwareand software under these circumstances, and how best to implement thedescribed functionality for each particular application.

As examples, the various illustrative logical blocks, modules, circuits,and algorithm steps described in connection with the embodimentsdisclosed herein may be implemented or performed with a digital signalprocessor (DSP); an application specific integrated circuit (ASIC); afield programmable gate array (FPGA) or other programmable logic device;discrete gate or transistor logic; discrete hardware components such as,e.g., registers and FIFO; a processor executing a set of firmwareinstructions; any conventional programmable software module and aprocessor; or any combination thereof designed to perform the functionsdescribed herein. The processor may advantageously be a microprocessor,but in the alternative, the processor may be any conventional processor,controller, microcontroller, or state machine. The software modulescould reside in RAM memory, flash memory, ROM memory, EPROM memory,EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or anyother form of storage medium known in the art. The processor may residein an ASIC (not shown). The ASIC may reside in a telephone (not shown).In the alternative, the processor may reside in a telephone. Theprocessor may be implemented as a combination of a DSP and amicroprocessor, or as two microprocessors in conjunction with a DSPcore, etc.

The previous description of the preferred embodiments is provided toenable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

1. An apparatus for a wireless communication system supporting LargeArea Synchronized-Code Division Multiple Access (LAS-CDMA)transmissions, the transmissions using LS codes for spread-spectrummodulation, the apparatus comprising: means for determining a size of aninterference free window (IFW); means for determining a plurality ofsubsets of LS codes, each subset comprising a number of LS codes as afunction of the IFW; means for assigning a first of the plurality ofsubsets of LS codes to a first portion of the wireless communicationsystem; means for assigning a second of the plurality of subsets of LScodes to a second portion of the wireless communication system; andmeans for identifying mobile stations within the first portion of thewireless communication system with LS codes from the first of theplurality of subsets; and means for identifying mobile stations withinthe second portion of the wireless communication system with LS codesfrom the second of the plurality of subsets.
 2. The apparatus of claim1, wherein the means for determining the plurality of subsets of LScodes comprises: means for determining a number of subsets of LS codesfor assigning within the wireless communication system; means fordetermining the first subset of LS codes having null cross-correlationwith respect to each other; and means for determining the second subsetof LS codes having null cross-correlation with respect to each other. 3.The apparatus of claim 1, wherein a cross-correlation of the LS codeswithin the first of the plurality of subsets is null within the IFW, andwherein the cross-correlation of the LS codes within the second of theplurality of subsets is null within the IFW.
 4. The apparatus of claim1, wherein the means for determining the plurality of subsets furthercomprises: means for determining seed pairs given as: (C1; S1); and (C2;S2); and means for determining a plurality of LS codes by application ofa formula given as: (C1   C2; S1   S2) (C1 −C2; S1 −S2) (C2   C1; S2  S1) (C2 −C1; S2 −S1),

wherein a negative indicates a binary complement of an original element.5. The apparatus of claim 4, wherein the plurality of subsets of LScodes is at least three.
 6. The apparatus of claim 1, wherein the firstand second portions correspond to first and second cells, respectively,in the wireless communication system.
 7. The apparatus of claim 1,wherein the IFW is determined based on delay profiles of mobile stationsin the first and second portions of the wireless communication system.8. An apparatus for a Large Area Synchronized-Code Division MultipleAccess wireless communication system, the apparatus comprising: meansfor transmitting a first communication within a first cell, the firstcommunication identifying at least one mobile station within the firstcell by a first LS code within a first subset of LS codes; and means fortransmitting a second communication within a second cell, the secondcommunication identifying at least one mobile station within the secondcell by a second LS code within a second subset of LS codes; wherein across-correlation between any two LS codes within the first subset isnull within an interference free window, and the cross-correlationbetween any two LS codes within the second subset is null within theinterference free windows wherein the first and second subsets of LScodes are part of a set of LS codes and are defined by the interferencefree window, and wherein the set of LS codes comprises 128 LS codes, andthe interference free window is equal to [−1, +1]and corresponds to 64available LS codes for each of the first and second subsets.
 9. Theapparatus of claim 8, wherein a correspondence between the interferencefree window and a number of available LS codes for each subset is basedon an arborescence structure.
 10. A computer readable storage mediumencoded with computer executable instructions to be executed by aprocessor in a wireless communication system supporting Large AreaSynchronized-Code Division Multiple Access (LAS-CDMA) transmissions for:determining a size of an interference free window (IFW); determining aplurality of subsets of LS codes, each subset comprising a number of LScodes as a function of the IFW; assigning a first of the plurality ofsubsets of LS codes to a first portion of the wireless communicationsystem; assigning a second of the plurality of subsets of LS codes to asecond portion of the wireless communication system; identifying mobilestations within the first portion of the wireless communication systemwith LS codes from the first of the plurality of subsets; andidentifying mobile stations within the second portion of the wirelesscommunication system with LS codes from the second of the plurality ofsubsets.